The development of prodrugs—chemically modified versions of the pharmaceutically active agent that must undergo transformation in vivo prior to exerting their pharmacological or therapeutic action—has become one of the strategies to improve the physicochemical, pharmacokinetic and/or pharmacodynamic properties of pharmaceutically active agents, and thereby enhance their efficacy and/or reduce their side effects. For example, prodrugs provide possibilities to overcome various barriers to drug formulation and delivery such as poor aqueous solubility, physical and/or chemical instability, insufficient absorption, rapid pre-systemic metabolism, inadequate brain penetration, toxicity and/or local irritation. Prodrugs can also improve drug targeting and duration of action.